It is well known that ATM networks are subject to congestion when the traffic offered to the network exceeds the capacity of the network. Such congestion conditions need to be controlled in order to guarantee, for each ATM connection, the Quality of Service (QOS) negotiated between the communicating end systems and ATM network during call establishment.
Recommendations for traffic control and congestion control in ATM networks are contained in ITU-T (Telecommunication Standardization Sector of the International Telecommunication Union, formerly CCITT) Recommendation I.371 and in the ATM Forum UNI 3.0 Specification. One of the congestion control mechanisms defined in these recommendations is the Explicit Forward Congestion Indication (EFCI). This is an optional mechanism that may be used to assist the network in the avoidance of and recovery from a congested state. A network element (e.g., a cell buffer queue feeding an internodal link) that is in a congested state may set the EFCI in the header of cells passing through it to indicate the existence of congestion (in the direction of the cell flow) to the destination end system. Cells when first introduced into the network by the source end system have the value of this indication set to "congestion not experienced." Also, a network element that is not congested is not allowed to modify the value of this indication. Hence, if a cell encounters at least one network element that is congested along the path of its ATM connection, the destination end system is informed of this congested state via the EFCI in the cell header. Although not specified in either I.371 or the ATM Forum UNI 3.0 Specification, it is suggested that the EFCI may be used to implement higher layer protocols in the end systems that adaptively lower the cell rate during congestion. Studies of feedback-based congestion control mechanisms that may make use of EFCI show promise of achieving effective, yet simple, congestion control in ATM networks.
Despite the simplicity of an end-to-end congestion control mechanism based on EFCI, there is concern that such a mechanism would not provide effective control over misbehaving or malfunctioning end systems. That is, suppose the end systems for an ATM connection do not react appropriately to the indication of congestion carried by EFCI (e.g., do not reduce the cell rate for the connection). Other end systems that are appropriately reacting to congestion indications may be at a disadvantage and, in some cases, may not even obtain the QOS guaranteed to their ATM connections. This has led to other traffic management proposals that require the generation of special traffic management (TM) cells from different points in the network (edge and/or intermediate) back toward the source end system. These cells effectively form a backward indication of congestion.
The advantage of using network (rather then end system) generated TM cells is that the network does not have to depend on the destination end system to properly signal the congestion indication back to the source end system. Moreover, the traffic policing (referred to as User Parameter Control) functions, usually performed at the source edge node of the network, can use these backward TM cells to implement protective traffic management mechanisms.
However, the generation of backward TM cells for each ATM connection represents a considerable complexity burden to the processing of ATM cells at edge and intermediate nodes in the network. Moreover, unlike the EFCI indications which are forwarded in cell headers of normal end system traffic, the backward TM cells consume link bandwidth resources and may be generated from multiple points in the network. This burden of backward indications is particularly evident when one is using a single ATM Virtual Path Connection (VPC) to carry multiple (say, 100s) of Virtual Channel Connections (VCCs). A single backward indication on a VPC across an ATM interface between two ATM networks may require the generation of a separate backward TM cell for each of the multiplexed VCCs.
Thus, there is a need for an efficient network traffic management method and system that combines an end-to-end closed-loop control method with the protection capabilities of a subnetwork-level feedback and policing function in a cell relay communication network.